Thermal stabilization of polybenzimidazole fiber fabrics

ABSTRACT

A PROCESS FOR PRODUCING A THERMALLY STABLE POLYBENZIMIDAZOLE FIBER FABRIC BY TREATING THE SCOURED CLEAN RELAXED FABRIC WITH A SILICONE EMULSION AND THEN HOLDING THE FABRIC AT FIXED LENGTH AND WITH DIMENSIONS WHILE EXPOSING IT TO A TEMPERATURE OF ABOUT 950* F.

United States Patent O No Drawing. Filed Feb. 28, 1973, Ser. No. 336,584 Int. Cl. B29d 7/22 US. Cl. 117-7 1 Claim ABSTRACT OF THE DISCLOSURE A process for producing a thermally stable polybenzimidazole fiber fabric by treating the scoured clean relaxed fabric with a silicone emulsion and then holding the fabric at fixed length and with dimensions while exposing it to a temperature of about 950 F.

BACKGROUND OF THE INVENTION This invention relates to a process for treating polybenzimidazole fibers. More particularly, this invention concerns itself with a process for preventing thermal shrinkage of fabric materials made from polybenzimidazole fibers.

The thermal shrinkage of synthetic fabrics during their manufacture as well as during certain uses, for example, under exposure to high temperatures or to fire, constitutes a serious problem in the textile field. Numerous methods have been suggested in attempting to solve this problem. However, although some of these attempts have been successful, the resultant fabric material loses its softness and flexibility due to the agents utilized in preventing theshrinkage of the fabric material.

A problem often encountered in the utilization of synthetic fabrics, especially of the polybenzimidazole type, concerns itself with the fact that these materials, upon exposure to an elevated temperature environment, shrink and become very stiff and boardy unless they receive a suitable pretreatment.

With the present invention, however, it has been found that the tendency for polybenzimidazole fiber textiles to shrink during exposure to high temperatures or fire can be overcome with no loss of flexibility in the final product. In this invention, the polybenzimidazole fabrics, in a relaxed condition, are first scoured with a detergent solution, rinsed, and dried. The fabric is then treated with a silicone solution, held to fixed length and width dimensions by being mounted on a rigid frame and then heat treated or set. The heat treated fabric is then removed from the frame and machine washed and dried. The resultant product is soft and flexible. It can be re-exposed to heat at temperatures of from 400 to 1100 F. while in a relaxed condition with negligible shrinkage and no loss in softness or flexibility.

SUMMARY OF THE INVENTION In accordance with this invention it has been found that the successful pretreatment of polybenzimidazole fiber textiles can be accomplished by a process which involves treating the relaxed fabric with a silicone solution followed by a heat treatment while the fabric is held under fixed length and width dimensions. In brief, the fabric material is first scoured and dried. It is then soaked in a relaxed condition with a silicone emulsion and dried while in its relaxed state. The silicone-treated fabric is then mounted on a pin frame at fixed length and width, and exposed to a temperature of 950 F. for a period of time. The heattreated material is then removed from the frame. The resulting fabric is soft and flexible and, if reexposed to elevated temperatures while relaxed, the shrinkage is negligible and the flexibility is retained. Continuous processing of long lengths of fabric can be accomplished by use of a conventional textile finishing mill tenter frame. Here the silicon pre-treated fabric is held at fixed length and width, on the tenter frame while passing continuously through the tenter frame oven, whereby each increment of cloth attains the requisite temperature for the proper period of time to assure fabric setting.

Accordingly, the primary object of this invention is to provide a process for pre-treating synthetic textile fabrics of the polybenzimidazole type.

Another object of this invention is to provide a process for inhibiting the shrinkage of polybenzimidazole textile fibers while still retaining fabric flexibility.

Still another object of this invention is to provide a process for producing a thermally stable polybenzimidazole fiber fabric that will not shrink or distort when exposed relaxed to temperatures in the range of about 400 to 1100" F.

The above and still other objects and advantages of the present invention will become more readily apparent upon consideration of the following detailed disclosure thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention provides a process for pre-treating polybenzimidazole fiber fabrics of the type sold under the name PBI Fabrics, and made by Fabric Research Laboratories of Dedham, Mass. The process provides a means for producing thermally stable fabrics which do not shrink and retain their flexibility even when exposed to a high temperature environment up to 1100" F. The process consists of first treating the fabric material with a silicone emulsion followed by exposing the fabric, suitably held in a pin frame at fixed length and width so as to prevent the shrinkage of the fabric, to such a temperature and for such a time as would cause the fabric to shrink if it were not so restrained.

For the purpose of illustrating the invention, the following detailed example is presented.

EXAMPLE A relaxed panel, 60 inches in length and 24 inches in width of polybenzimidazole fiber fabric is first scoured for 30 minutes at 60 C. in an aqueous solution of 2 grams per liter of Tergitol non-ionic NPX detergent, chemically defined as a nonylphenol and 0.5 gram per liter of sodium tetrapyrophosphate, followed by a water rinse. The panel is then desized by a further immersion for 30 minutes at C. in an aqueous proteolytic enzyme solution of 1.5 grams per liter of Rhozyme PF mix 564, suitably buffered. The mix 564 is made by Rohm and Haas Co. of Philadelphia and is chemically defined as a proteolytic enzyme. The fabric is then rinsed again for 30 minutes at 50 C. in several changes of clear water and air-dried at C. The scoured and dried relaxed fabric is then soaked in a 10 percent by volume solution of a 5:1 mixture of (1) silicone emulsion ET-4-0206 made by the Dow Corning Corporation of Midland, Mich., and defined chemically as a Dimethyl polysiloxane; and (2) catalyst 22A made by the Dow Corning Corporation of Midland, Mich., and defined chemically as a metal organic salt. The silicon treated fabric is then allowed to drain for 2 minutes, then dried and cured relaxed in air at 300 F. for 15 minutes. The dried, silicone treated fabric is then mounted on a pin frame and is held at a fixed length of 60 inches and a fixed width of 24 inches in a manner such that the fabric panel is straightened by tension but the individual fabric yarns are not stretched in either direction, i.e., no slac is evident. It is then exposed whilst still mounted on the pin frame to a temperature of 950 F. for 45 seconds in a circulating hot air oven. The heat-treated panel is then removed from the frame and machine-washed and tumbledried. The resulting fabric is soft and flexible. When exposed to temperatures in the range of 400 to 1100 F., when relaxed, the resulting fabric displays negligible thermal shrinkage while retaining softness and flexibility.

Various tests were carried out on the heat-treated fabric and on the untreated control fabric; results of the tests are given in Table I.

TABLE I [Comparison of mechanical properties of thermally stable p.b.i. fabric with those of untreated control] Thermally stable Untreated Property fabric control Construction 3 (warp Xfilling threads per inch) 64x61 63x60 Weight 3 (oz./sq. yd.) 6. 5 6. 2 Thickness 3 (inch) 0. 0165 0. 0165 Tensile strength 1 (lbs/inch):

Warp 111 125 101 116 a p 21. 4 35. 2 Fill 18. 7 31. 4 Tearing strength 1 (ibs.):

254 195 234 134 Moisture regain 1 (percent at 70 F., 65 percent RH) 9. 6 10. 8 Shrinkage on exposure to 1,100 F. [or 60 see. 1

(percent) 4 9. 6 5 35 Strength retention 1 (percent of 70 F.

strength):

700 F.; 15 min 60 48 800 F.; 15 min 35 15 900 F.; 15 min 6 1 Each value is the average of 2 determinations.

2 Each value is the average of 8 determinations.

a Each value is the average of many determinations. 4 Fabric remains flexible.

Fabric becomes stiff and embrittled.

The construction of the treated fabric is essentially identical to that of the untreated control; this is to be expected from the nature of the restraints imposed during the heat treatment. The small increase in weight/unit area of the treated fabric is probably caused by residue from the silicone emulsion pretreatment, and might be expected to decrease on further washing. The tensile strength of the treated fabric is approximately less than that of the untreated control, and the rupture elongation is reduced from approximately 30% to The tearing strength is approximately 8 lbs. in the treated state; some reduction from the control values of 20.5 lbs. (warp) and 11.5 lbs. (fill) is to be expected from the observed reductions in rupture load and elongation, and the slight increase in the fabric density, but the magnitude of the reduction is surprising. It should be noted, however, that the tearing strength of the treated polybenzimidazole fabric is greater than that of a flame retardant treated cotton flight suit fabric (warp 5.3 lbs.; filling 7.9 lbs.) and compares favorably with that of a 4.2 oz./sq. yd. Nomex" aromatic polyamide flight suit fabric (warp 10.7 lbs.; filling 9.6 lbs.). The flexural rigidity of the fabric is increased by a small amount on treatment, but preliminary results indicate that the flexural rigidity decreases on further washing and that a fabric very similar in hand to the original fabric can be produced. The moisture regain at 70 F. and RH is reduced a little by treatment; again this reduction would probably disappear on further washing.

The shrinkage on exposure to 1100 F. is drastically reduced and the heat-treated fabric is remarkably flexible after high temperature exposure in contrast to the control, which becomes very stiff and brittle. The treated fabric does not shrink more than 10% on exposure to the flame of a propane torch, and even when heated to red heat it does not burn in air, nor does it lose flexibility. The strength reduction of the treated fabric on prolonged exposure to high temperature is very good.

In general, the process of this invention yields a fabric with good hand and mechanical properties at room temperature, and greatly improved thermal stability and flexibility and strength characteristics at high temperature; the detrimental effects of the treatment on the properties of the fabric at room temperature are quite small and are well within accept-able limits. In particular, the tensile strength and the moisture regain are not significantly reduced.

While the principle of this invention has been described with particularity, it should be understood that various modifications can be made Without departing from the spirit of the invention, the scope of which is defined by the appended claims.

What is claimed is:

1. A process for treating a polybenzimidazole fiber material comprising the steps of (1) soaking said material in a 10 percent by volume of a silicone solution composed of a 5: 1 mixture of dimethyl polysiloxane and a metal organic salt catalyst; (2) drying said soaked material in air at about 300 F. for about 15 minutes; (3) holding said material under tension at a fixed length and width dimension while simultaneously exposing it to a temperature of about 950 F. for about 45 seconds; and (4) relieving said material from any stresses that may have developed by holding the fabric under fixed length and width dimension.

References Cited UNITED STATES PATENTS OTHER REFERENCES Fridrichsen, A. F., Def. Pub. of Serial No. 618,431 filed Feb. 24, 1967, published in 860 CG. 1008, Mar. 25, 1969, T860,016.

WILLIAM D. MARTIN, Primary Examiner S. L. CHILDS, Assistant Examiner US. Cl. X. R.

il7 l l9.6, 138.81R, 16'1ZA 

